The research involves an in-depth study of the transport mechanisms responsible for, and the factors that regulate, net potassium absorption and secretion by mouse medullary and cortical thick ascending limbs of Henle. Potassium transport by thick ascending limbs may contribute importantly to renal K+ excretion either directly or through their contribution to the process of renal medullary potassium recycling. This latter recycling process is believed to establish conditions (i.e., medullary K+ concentration) that make it possible for distal nephron segments to secrete potassium following increases in potassium load during periods of normal or increased K+ intake and to absorb (or not excrete) potassium after potassium depletion. Furthermore, Na+, Ci- and K+ transport are interrelated processes in thick ascending limbs and may be regulated in parallel (as they are in the cortical collecting duct). Most studies will employ in vitro perfusion of isolated thick ascending limb segments and involve measurement of both unidirectional tracer and net chemical K+(Rb+) fluxes, as well as a number of electrophysiological parameters (membrane conductances, and electrochemical gradients for K+) relevant to K+ transport. In addition, Na-K-ATPase activity, the energy source for active K+ transport, will also be measured in single isolated segments. These methodologies will be employed to: (a) characterize the membrane transport processes that mediate active K+ secretion/absorption in both medullary and cortical thick limb segments from mice on a standard K+ intake; (b) assess the effects of K+ loading and depletion on net K+ and NaCl transport in these nephron segments; (c) identify the specific transport processes involved in any modulations in ion transport induced by K+ loading/depletion; (d) evaluate the effects of hormonal factors that could play a role in modulating K+ transport in thick limbs, including ADH, prostaglandin E2, Beta-adrenerigc agents and mineralocorticoids; and (e) examine the effects on K+ transport of several non-hormonal factors (pH, osmolality, and external K+ concentration) that modulate NaCl absorption in medullary thick limbs. These studies will provide basic insights into the transport mechanisms that mediate active K+ secretion/absorption in thick ascending limbs and into the factors that modulate these processes and should provide a more basic understanding of the role of this nephron segment in renal K+ excretion.